It was not terribly surprising that Romney would, on the eve of the election, toss aside the antiabortion positions he cultivated during the Republican primaries; lately, he has reversed himself more often than a parking-lot attendant.

The surprise has been the reaction from conservatives. “No alarm bells here,” the Family Research Council’s president, Tony Perkins, proclaimed to Talking Points Memo. Perkins said he had been assured by Romney’s campaign that the answer was a product of “the way the question was asked.”

Romney later clarified his remarks, stating that he remained antiabortion. Still, the green light given by a top group on the religious right to Romney’s recasting of his abortion position is typical of recent weeks. Conservatives have been sitting silently — approvingly, even — as Romney makes his late lunge for the center. For a movement that has prided itself on being ideologically pure, this is a decidedly pragmatic turn.

Necessity, it seems, is the mother of reinvention.

Key to the success of Romney’s Etch a Sketch movement has been the cooperation of conservatives, who have been unusually docile in the face of the candidate’s heresies: pledging not to enact a tax cut that adds to the deficit, promising not to decrease the share of taxes paid by the wealthy, vowing not to slash education funding, praising financial regulations, insisting that he would make health insurers cover preexisting conditions and disavowing his earlier claim that 47 percent of Americans are parasites living off of the government.

At Tuesday night’s debate, Romney continued his sprint to the center. He took pains to say he is “so different” from George W. Bush. He asserted that “every woman in America should have access to contraceptives,” and, on immigration, he said the children of illegal immigrants “should have a pathway to become a permanent resident of theUnited States.” After a primary battle in which GOP candidates tried to out-tough each other on immigration, Romney said that he was in agreement with President Obama and that “I’m not in favor of rounding up people.”

The conservatives’ complicity seems to be driven by two things: a belief that Romney’s moves to the middle are mere feints, shifts more in tone than in substance; and an acceptance that Romney’s rhetorical reversals are necessary if he is to deny Obama a second term.

“I hear all this as tonal,” Grover Norquist, the Republican purity enforcer and keeper of the antitax pledge, told me. Romney’s new pledge that his tax cuts wouldn’t increase the deficit, for example, could be honored simply by using an alternative accounting method, known as “dynamic scoring,” that conservatives favor. “You’re now in the general election and you’ve already convinced conservatives why they should vote for you,” Norquist said of Romney. “You’re now talking to undecided voters, who have a completely different set of issues.”

Had Romney tried to moderate his positions over the summer, conservatives still suspicious from the primaries would have called him a turncoat, which would have depressed Republican turnout. But two weeks ago, polls showed that Romney’s “severely conservative” candidacy was heading to a seemingly inevitable defeat. It was that sense of desperation that gave Romney room to make his late break for the center, because conservatives were forced to accept that even a squishy and ideologically suspect President Romney would be preferable to Obama.

For example, Chris Chocola, president of the Club for Growth, which has worked to defeat insufficiently conservative officials in Republican primaries, gave Romney room to maneuver. “We tend to recognize the political realities,” he told Politico the day after the Denver debate, adding that “when it comes to the issues that the Club focuses on, Romney is 1,000 percent better than Obama.”

Rank-and-file Republicans seem inclined to follow the opinion makers’ lead in cutting Romney slack as he makes his late move to the middle. In Washington Post-ABC News polling, Romney’s support improved among self-identified Republicans, from 90 percent on Sept. 29 to 93 percent on Oct. 13. The number of Republicans saying they were very enthusiastic about him climbed to 59 percent from 48 percent. He suffered no attrition among self-described conservatives.

It has been a rare outbreak of common sense in the conservative movement. Romney should enjoy it while it lasts.